Recent research has established that the eye's focusing efficiency varies greatly under normal viewing conditions and that the accuracy of steady-state accommodation depends on characteristics of both the stimulus and the observer. While accommodation is usually accurate for bright, patterned stimuli, it is progressively biased toward the observer's characteristic dark focus (resting state) whenever stimulus luminance or contrast is reduced. Other research has shown that neonates and some amblyopic adults are incapable of accurate accomodation for any stimulus. This investigation will use a comparative approach to analyze the interactions of organismic and stimulus factors in determining the eyes' focus. Unobtrusive techniques, including laser refraction and photorefraction, will be used to study the focusing behavior of four primate populations: (1) normal human adults, (2) amblyopic human adults, (3) human infants; and (4) macaque monkeys. Accommodative responsiveness of all groups will be measured under stimulus conditions designed to reveal the effectiveness of spatial luminance contrast, chromatic contrast, and binocular fusion as accommodative stimuli. Comparison of results obtained from infants and amblyopes with those from normal subjects will provide new insights regarding the sensory processes that control accommodation. Studies of accommodation in the macaque should establish its utility as an animal model for further study of the neural processes underlying accommodative control.